For users that need extra power and features the following plugins are available.
Multi-threaded sending: ideal for those who want to send a high volume of newsletters in a short time frame. With a use of queue mechanism each campaign is executed in several parallel threads.
Import subscribers from an external database: You can setup connections to external databases, map your subscriber fields, save these connections and have them executed manually or synchronized with a cron job.
Feedback loop processing: for heavy senders who are registered with ISPs. You can process the complaints that ISPs send to your designated mailbox. Details
Opt-in by email: one can subscribe just by sending you an email. Details